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Late Glacial and Holocene depositional history in the eastern part of the Szczecin Lagoon (Great Lagoon) basin—NW Poland
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Quaternary International 130 (2005) 87–96
Late Glacial and Holocene depositional history in the eastern part of the Szczecin Lagoon (Great Lagoon) basin—NW Poland ! Ryszard K. Borowka*, Andrzej Osadczuk, Andrzej Witkowski, Brygida Wawrzyniak-Wydrowska, Tomasz Duda University of Szczecin, Institute of Marine Sciences, Wa) ska 13, Pl–71-415 Szczecin, Poland Available online 2 July 2004
Abstract Analyses on 27 sediment cores taken from the bottom of the Szczecin Lagoon allowed environmental reconstruction of the postglacial main stages of basin development, based on detailed sedimentological, geochemical, diatomological and malacological studies of selected key cores. Studies revealed that during the Late Glacial and Holocene this area developed in several stages. In the Late Glacial the whole study area constituted a low alluvial plain. At the turn from Younger Dryas to Holocene the alluvial plain was cut through by the Odra river to a level of 10–11 m below sea level (b.s.l.). Along with the first phases of the Holocene marine transgression at the southern Baltic Sea’s coasts the accumulation of the limnic-swampy deposits began in this lower part of the Odra valley. At ca. 6–6.5 ka BP the transgression proceeded and Littorina Sea waters flooded the area. At that time the Szczecin Lagoon constituted a marine embayment in which series of sands, partly rich in malacofauna, was deposited. The development of the Swina barrier resulted in the isolation of the embayment from the direct inflow of Baltic Sea waters. r 2004 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Holocene development of the southern Baltic Sea shows distinct differences in comparison to the other areas of the Baltic, e.g. isostatic rebound processes at the Scandinavian coasts, which are absent in the Southern . Baltic Sea (e.g. Svensson, 1991; Bjorck, 1995). Thus the southern coasts of the Baltic Sea usually show sedimentary sequences reflecting marine transgressions. Beginning from the onset of the Boreal and Atlantic Chronozones the global eustatic sea level rise caused an inflow of marine waters. This resulted in changes of the water chemistry and in a general water level rise. The rising water level led to flooding of the former terrestrial (lacustrine) environment and the shores were translocated towards south, beginning from ca. 7000 BP. This sequence of events is typical for the Pomeranian Bay and the neighbouring basins of the lower Odra Valley and the Szczecin Lagoon (Kolp, 1990; Kliewe and Janke, 1991; Kramarska, 1999; Broszinski, 2002). The Szczecin Lagoon (Fig. 1) together with the Great Da) bie and Small Da) bie lakes to the south constitutes a *Corresponding author.
system of extensive sedimentary basins in which sedimentation of allochthonous and autochthonous deposits took place during most of the last millenia. These deposits result from terrigenous fluvially transported material and in situ produced biogenous and chemogenous sediments. The major source of the terrigenous material was the Odra River. However, during storm-induced high levels of the Pomeranian Bay also the Swina, Dziwna and Peene Rivers transported sediments into the lagoon. The Szczecin Lagoon area is primarily the valley floor of the deglacial Odra River valley which, due to the post-glacial transgression, was transformed into a lagoon when the spits along the present coast of the Baltic Sea were formed. Earlier geological studies of the Szczecin Lagoon ! (e.g. Wypych, 1980; Leipe et al., 1998; Borowka et al., 1999a–c; Muller, . 2001) were principally based on analyses of solitary and often randomly located cores. The present paper is based on the results of a comprehensive geophysical survey and coring programme of the Lagoon, aiming towards the recognition of depositional sequences with a time span from lateglacial to the present.
1040-6182/$ - see front matter r 2004 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2004.04.034
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Fig. 2. Enlarged section of the seismoacoustic profile from the NE part of the study area (for location see Fig. 1).
malacological and geochemical analyses and for 14C dating. Detailed information on 14C dating is provided in Table 1.
3. The study area
Fig. 1. Location of the study area. Indicated are positions of the seismoacoustic profiles and of vibrocores.
2. Materials and methods Seismoacoustic studies of the Szczecin Lagoon sediments were done by means of the sub-bottom profiler ‘‘Seabed Oretech 3010-S’’ at 5 kHz. Altogether 18 seismo-acoustic profiles, 200 km in total length, were accomplished (Figs. 1 and 2). Based on these results 27 sites were chosen for vibrocoring (Fig. 1). Vibrocores were taken by means of a VKG-3 device, 4 m long and 9.1 cm of internal diameter. The cores 3–4 m in length were cut into 1 m intervals and stored at a temperature of 3–4 C. In laboratory conditions all cores were documented photographically and their lithology was described macroscopically. Included in the description were the colour of the sediments, structural and textural features, organic matter content and the presence of fossil macro-remnants. Sub-sampling was done for granulometric, diatomological, palynological,
The investigations were carried out in the eastern part, deep-water part of the Szczecin Lagoon. The Szczecin Lagoon is a shallow water body with a surface area of 687 km2. The recent depth of the lagoon does not exceed 8.5 m and has an average of c. 3.5 m. Almost half (48.3%) of the lagoonary surface area has water depths ranging from 4 to 6 m. The northern boundary of the lagoon, which also separates it from the Baltic Sea, is the Swina Bar. This bar complex is made up of Holocene beach deposits and deltaic deposits of the Swina River. In the south the lagoon is gradually getting narrower (Fig. 1) forming the ! so-called Roztoka Odrzanska (Odra mouth) and finally the Lower Odra Valley.
4. Results The total number of studied cores amounted to 27. Results of these studies show that the sedimentary infill of the Szczecin Lagoon is composed of several depositional units, indicating different and changing depositional environment. The following depositional series can be distinguished in stratigraphic order: fluvial, limnic-swampy, marine and lagoonal. The best examples of this development and succession were observed in cores 35/99, 42/99 and 3/96 (Figs. 3 and 4).
Table 1 Radiocarbon dates of limnic and swampy deposits from the Szczecin Lagoon Sample name
(2/98/0.64–0.75) (2/98/0.75–0.81) (9/96/97–100) 26/56 26/65 26/69 28/57 28/59–60 29/26 29/31 35/41–42 35/46 35/50 36/23–24 36/27 36/48 37/53 39/61–62 42/30 42/36 42/41 42/44 42/45 43/10 43/12–13 43/32–33 47/34–35 47/48–49
Material
Radiocarbon age (14C yr B.P.)
Calibrated age (cal yr B.P.)
Calibrated age range (1 sigma) (cal yr B.P.)
Laboratory number
2/98 2/98 9/96 26/99 26/99 26/99 28/99 28/99 29/99 29/99 35/99 35/99 35/99 36/99 36/99 36/99 37/99 39/99 42/99 42/99 42/99 42/99 42/99 43/99 43/99 43/99 47/99 47/99
0.64–0.75 0.75–0.81 4.90–5.00 2.75–2.80 3.20–3.25 3.40–3.45 2.80–2.85 2.90–3.00 1.24–1.30 1.49–1.53 1.92–2.00 2.15–2.20 2.35–2.40 1.10–1.20 1.30–1.33 2.82–2.86 2.62–2.66 3.00–3.10 1.50–1.55 1.80–1.86 2.00–2.03 2.10–2.14 2.14–2.18 0.44–0.50 0.55–0.63 1.51–1.61 1.66–1.75 2.35–2.45
Peat Peat Organic mud Detritus gyttja Mudy gyttja Organic mud Organic mud Detritus gyttja Organic mud Detritus gyttja Peat Peat Detritus gyttja Peat Peat Organic mud Organic mud Detritus gyttja Detritus gyttja Detritus gyttja Peat Peat Muddy peat Detritus gyttja Detritus gyttja Organic mud Detritus gyttja Muddy gyttja
74107140 74307140 72507130 72407120 79907110 73307110 62307250 77007120 7050790 69307110 7030790 88107110 104207180 78707100 71707110 111007130 119007230 81807160 6850790 7230780 100307120 114507140 101807380 70807100 77707130 99807110 7260775 78507100
82357125 82657115 80607130 80607120 88507160 80907120 71257325 84907120 78557105 77607100 78507100 98757275 123007350 86657125 79507110 130557145 138757425 91007350 7685775 8100790 115007250 133507200 119507700 78807100 85557165 114357205 80707100 87257275
8360–8110 8380–8150 8190–7930 8180–7940 9010–8690 8210–7970 7450–6800 8610–8370 7960–7750 7860–7660 7950–7750 10150– 9600 12650–11950 8790–8540 8060–7840 13200–12910 14300–13450 9450–8750 7760–7610 8190–8010 11750–11250 13550–13150 12650–11250 7980–7780 8720–8390 11640–11230 8170–7970 9000–8450
Gd-15096 Gd-12233 Gd-15092 Gd-15103 Gd-12236 Gd-15101 Gd-16027 Gd-12234 Gd-12241 Gd-12244 Gd-12223 Gd-12224 Gd-15090 Gd-12242 Gd-12243 Gd-12245 Gd-15086 Gd-15078 Gd-12217 Gd-12222 Gd-12214 Gd-12220 Gd-16029 Gd-12235 Gd-15104 Gd-12238 Gd-12225 Gd-12226
6.64–6.75 6.75–6.81 9.90–10.00 8.70–8.75 9.15–9.20 9.35–9.40 8.52–8.57 8.62–8.72 6.96–7.02 7.21–7.25 6.93–7.01 7.16–7.21 7.36–7.41 7.05–7.15 7.25–7.28 8.77–8.81 8.48–8.52 8.77–8.87 7.55–7.60 7.85–7.91 8.05–8.08 8.15–8.19 8.19–8.23 5.76–5.82 5.87–5.95 6.83–6.93 7.58–7.67 8.27–8.37
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Fig. 3. Core logs 35/99 and 42/99: 1—shell layer; 2—algal gyttja with shells; 3—algal gyttja; 4—mineral gyttja; 5—silty gyttja; 6—sandy gyttja; 7— detritus gyttja; 8—peat; 9—sand; 10—silty sand.
Fig. 4. Core log 3/96 including results of the malacological analysis (for explanations see Fig. 3).
4.1. Late Glacial sediments 4.1.1. Unit A: fluvial environment The basement of the organic deposits filling in the Szczecin Lagoon basin is of fluvial origin. The coring equipment could not penetrate the full thickness of this unit but a data from older studies (e.g. Wypych, 1980) indicate that it reaches at least the depth of 24 m b.s.l. The total thickness could thus reach 14–16 m, possibly
with the lower part representing fluvio-glacial deposits. The maximum penetration at coring these sediments was 3.5 m. The sediments are well- to very well-sorted fine to medium sands, in the lower parts sometimes showing cross bedding. In the uppermost part the sand is more fine-grained and the beds show a horizontal lamination. Within some of the cores (e.g. 35/99; 42/99 and 47/ 99), the upper part of the unit A sands display numerous
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root horizons and charcoal intraclasts. In core (36/99, 37/99 and 42/99) the unit A fluvial sand is interbedded with a thin subunit (10–20 cm) of organic-rich, clayeymuddy sediments, interpreted to be of limnic-swampy origin. 14C dates (core 42/99) of this layer indicate that it . age. is of Allerod The morphology of the upper surface of the unit A fluvial sediments is variable (Fig. 5A). Its deepest position was recorded in the middle part of the Great Lagoon, where it appears at 10–11 m b.s.l. and in the north-western part, close to the border of the Little Lagoon. In the eastern and western part of the coast of the Great Lagoon the level of the upper boundary rises to 6–7 m b.s.l. The analysis of the seismo-acoustic data thus reveals the presence of an elongated basin with a relative height of up to c. 3 m within the upper surface of the fluvial sediments, which is interpreted as an erosional channel related to the lateral migration of the Pre-Odra River.
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4.2. Holocene sediments 4.2.1. Unit B: Limnic-swampy environment Organic deposits, though absent in some cores, drape the unit A top surface with a sediment thickness between 10 cm and 2 m. Spatial analysis of its thickness distribution shows that these sediments primarily fill the elongated depressions within the upper boundary of the fluvial unit A sediments, particularly in the middle part of the Great Lagoon (Fig. 5B). They are interpreted as ancient river beds of the Pre-Odra River. The unit B sediments are predominantly composed of gyttja, sometimes underlain or overlain by well decomposed peat with wood pieces. The organic matter content of the gyttja range from 20% to 80%, sometimes with a distinct portion of CaCO3. The gyttja display a freshwater molluscan fauna with the dominant taxa being characteristic for small lacustrine basins, e.g. Bithynia tentaculata, Valvata piscinalis, Lymnaea sp. and
Fig. 5. Szczecin Lagoon. Morphology of the top surface of the fluvial (Unit A), swampy-limnic (Unit B), brackish-marine (Unit C) and lagoonary series (Unit D).
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Pisidium sp. Some fish remnants were also sporadically identified within the sediment. An abundant diatom flora was recorded (Fig. 6) predominated by the epiphytic taxa representing genera of Cocconeis, Epithemia, Fragilaria and Gomphonema. Their predomination indicate the existence of shallow freshwater bodies with abundant macro-plants. Furthermore, the dominance of benthic taxa also indicates a shallow water environment with good light conditions. The frequency (up to 6%) of brackish-water taxa increases in the upper part of the unit, e.g. represented by Fragilaria schulzii, Navicula salinarum and Melosira nummuloides. In more shallow parts of the basin, unit B is represented only by peat layers up to ca. 2 m in thickness. Peat type varies, with carex and carex-reedy peat in the basal part of the profile and woody and woody-carex peat in the upper part. Based on sediment types and macro-fossil content, unit B is interpreted as deposited in a varying swampy to limnic environment. The deposition started as indicated from 14C dating (Fig. 3), in the beginning of the Holocene and lasted up ! to ca. 6200 BP (Borowka et al., 2001). Existing data indicate their formation to start after a distinct environmental shift from fluvial conditions to shallow limnic conditions. Only one core (43/99, Fig. 1) records fluvial sediment interbedding (c. 1 m sands) within the unit B limnic sediments. 4.2.2. Unit C: marine environment The base of the unit C sediments is marked by a distinct erosional discontinuity and occurs in almost all cores taken from the deeper part of the Great Lagoon. They are characterised by increased contents of the coarse fraction and relatively good sorting. The thickness of the marine unit C sediment is variable, but it rarely exceeds 1.5 m. Its greatest thickness was recorded in the northern part of the Great Lagoon (Fig. 5C); in core 31/99 (Fig. 1) it attained ca. 3 m without being
totally penetrated. The marine sediments were not encountered in the lateral, more highly siltated parts of the basin. With respect to lithology, the marine sediments are composed of sands and muddy sands. They differ from the unit A fluvial sands with respect to organic matter, Na, K, Ca, Mg and Fe contents (Fig. 6), finer grain-size and less sorting. The results from the malacological analyses are highly indicative of a marine environment. The mollusc fauna of the sandy sediments is dominated by Cardium glaucum, forming well developed populations with shell sizes larger than in the recent Pomeranian Bay. Accompanying species are Hydrobia ventrosa, H. ulvae and H. balthica (Fig. 4). A characteristic feature of the marine sediments is either a complete absence or a scarce, usually highly fragmented diatom flora (Fig. 7). This might be related to a high sedimentation rate of the sandy sediments and to current and wave action. The taxa found, e.g. Catenula adhaerens and Fragilaria fasciculata, are typical for a shallow marine littoral environment. 4.2.3. Unit D: Lagoon environment This unit differs from the marine unit with respect to a higher content of organic matter, to their chemical composition and the species composition of the molluscan fauna. Locally, its thickness exceeds 2 m in the middle part of the Great Lagoon, but it is very thin in the western part (Fig. 5D). The subfossil malacofauna of the lagoonary sediments is predominantly composed of freshwater taxa including Bithynia tentaculata, Valvata piscinalis, Viviparus sp., Pisidium sp., Unio sp., and Dreissena polymorpha. Presently, the latter species forms extensive colonies on some localities of the Great Lagoon bottom (e.g. core 35/99). The diatom flora (Fig. 7) is dominated by freshwater planktonic taxa such as Aulacoseira
Fig. 6. Distribution of diatom ecological groups in core No. 3/96. Solid area stands for %, dotted area stands for %.
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Fig. 7. Distribution of geochemical elements in core No. 3/96.
granulata, A. islandica and A. muzzanensis. Within the youngest deposits halophilous and brackish-water taxa appear, attaining percentages up to 9.6% and represented by taxa such as Diploneis smithii, Fragilaria sopotensis and Opephora olsenii. The lagoonary sediments show a distinct bipartition with respect to geochemical analyses (Fig. 6). Its upper part, up to half a meter in thickness, is characterised by markedly increased concentrations of heavy metals (Zn, Pb, Cu) and Mn. In comparison to the lower part there is a nine-fold increase in Zn and an almost five-fold increase in Cu content.
5. Discussion The analysis of the stratigraphic and spatial variability of the Great Lagoon sedimentary infill enables the distinction of several development stages of this depositional basin. Their chronostratigraphic position is based on numerous radiocarbon dates of the limnic-swampy ! deposits (Borowka et al., 2001). A summary of the paleogeographic development is presented in Fig. 8. . stage is represented by fluvial The Pre-Allerod sediments (unit A). Their origin is related to the development of the lower Odra River valley from the end of the last glaciation to the distinct climate . Within this period, a amelioration during the Allerod. series of six terrace levels in the area of the recent Odra River valley and the Szczecin Lagoon was formed. The highest is raised up to ca. 20 m a.s.l., whereas the lowest one is found at ca. 10 m b.s.l. (Karczewski, 1968; Duda, 1999). The age of particular terraces is not known yet. However, within terrace sediments at an altitude of ca.
8 m b.s.l., there are thin intercalations of limnic-swampy . deposits, dated to the Allerod. It is hard to reconstruct the late glacial bottom of the Odra River Valley and the course of the river bed itself. Paleohydrological studies (Kozarski and Rotnicki, 1977; Kozarski, 1983; Rotnicki, 1983, 1991) carried out for the Odra River catchment area and some of its tributaries revealed, that during the Late Glacial, large meandering river beds were formed within the bottom of some of the contemporary valleys. Compared to the Early and Middle Holocene they discharged several times higher magnitudes of water (Rotnicki, 1983, 1991). However, it cannot be excluded that the lower Odra was a braided river, similar to other rivers draining the northern slope of Pomerania (Florek, 1996). This problem thus remains an open question. . stage, it is known that some parts of From the Allerod the Szczecin Lagoon were covered by shallow water bodies and peat bogs (Fig. 8), giving rise to sediments interfingering with the predominantly fluvial A sediments. Simultaneously the surroundings of the Szczecin Lagoon were characterised by rather dense pine-birch forests and by patches of heliophilous plants. This is demonstrated not only from the results of paleobotanical analyses (Lata"owa, 1989, Lata"owa, 1999a, b), but also indicated by well developed and widely distributed ! covers of fossil soils on Wolin Island (Borowka et al., 1982, 1986, 1999a, b). The Younger Dryas stage is marked by a distinct acceleration of fluvial processes, and particularly by an aggradation of sandy deposits at earlier developed . limnic-swampy deposits in the Great covers of Allerod . Lagoon area. In cores 36/99 and 37/99, post-Allerod fluvial deposits attain a thickness exceeding 1.5 m. This
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Fig. 8. Szczecin Lagoon. Litho- and chronostratigraphy of the infilling deposits.
was probably related to the deterioration of climatic conditions during Younger Dryas, causing the disappearance of forest stands and acceleration of soil erosion processes (Lata"owa, 1999a, b). There are also indications on the intensification of periglacial and aeolian ! processes (Borowka et al., 1999a, b). During that period there was probably a distinct increase in the sediment load, transported by the Odra River. The Early and Middle Holocene stages encompass the time span from the beginning of the Preboreal to the second half of the Atlantic period (B10,250–6200 yr BP) at which the Great Lagoon area was predominantly filled in with limnic-swampy deposits (unit B; Fig. 8). It is highly likely that during that time the Pre-Odra was an anastomosing river, flowing through swamps and reedy areas, similar to the recent lower Odra.
The Late-Atlantic stage is marked by a phase of marine transgression (the Littorina transgression) in the Great Lagoon area. Almost the whole lagoon constituted an open marine bay at that time. It extended southward into the lower Odra River valley up to a place where the recent town Szczecin is located (Bor! owka and Duda, 2002) and penetrated the mouths of smaller rivers discharging into the Szczecin Lagoon, e.g. Uecker valley in the area of Ueckermunde . (Bramer, 1978). The water salinity was higher than in the present Pomeranian Bay, as indicated by preserved shells of Cardium glaucum, reaching sizes characteristic for water ! salinity values higher than 6–7% (Borowka et al., 2000). The duration and rate of this transgression in the area of the Pomeranian Bay and Szczecin Lagoon is not satisfactorily known. It cannot be excluded that it was a disastrous event, as suggested by Rosa (1963). It is possible that, during extremely strong storms, a sand bar between the Odra Bank and eastward up to the region of Ko"obrzeg was disrupted and destroyed (Mojski, 1995; Kramarska, 1999). Radiocarbon dates of limnic-swampy deposits from the Pomeranian Bay (Kramarska, 1999) and from the Szczecin Lagoon ! (Wypych, 1980; Borowka et al., 2001), showing very similar ages of these deposits in both areas, indicate a relatively fast rate of this transgression. The Late-Holocene stage, encompassing the Subboreal and the Subatlantic periods began with the isolation of the Szczecin Lagoon from direct marine influences. Intensified abrasion processes at morainic cliffs of Usedom and Wolin Islands resulted in a rapid growth of bars and the development of a sand barrier in the area of the recent Swina Gate (Keilhack, 1912; Prusinkiewicz and Norys!kiewicz, 1966). Simultaneously the formation of the Swina back-delta started behind the barrier. Its submerged part, the so-called Wyskok Krzecki, extends far into the lagoon’s interior and is dipping with a relatively steep slope to the depth of 6.5–7 m. The isolation of the Great Lagoon from direct marine influences resulted in a change of the depositional processes. This is indicated in the deposited sediments (unit D, Fig. 8) by an increase of the organic matter content and the replacement of marine molluscs by ! freshwater taxa (Borowka et al., 2000). Lagoonary sediments deposited during that time also contain a ! predominantly freshwater diatom flora (Borowka et al., 1999). The accumulation of this type of deposits has continued in the Great Lagoon until present times. The sedimentation rate is highly variable and ranges from ca. 20 cm per century in the Little Lagoon (Leipe et al., 1995) to ca. 50 cm in the eastern part of the Great ! Lagoon (Borowka, 2001). No deposition or very low deposition rates are characteristic for regions neighbouring the western coast of the Great lagoon, which are located on a distinct plateau on top of the fluvial unit A sediments.
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6. Concluding remarks The present geological studies have shown that the sediments filling the deep-water part of the Szczecin Lagoon are deposited on two morphological levels. . lacusOlder sediments, which encompass (e.g.) Allerod trine series are located on the higher level, c. 7–8 m b.s.l. Limnic and swampy sediments were initially deposited only at the lower terrace within the period of 10 to 6–6.5 ka BP. Later on, at ca. 8 ka BP, the swampy sedimentation inundated also the higher level. Beginning from this time the whole Great Lagoon constituted a vast and flat swampy plain, cut by the anostomosing Odra River. At c. 6–6.6 ka BP the whole low lying area was affected by a marine transgression and for a short period an open marine embayment existed, characterised by salinity values higher than the recent Pomeranian Bay. However, this embayment was rapidly isolated from the direct inflows of the Baltic Sea due to build-up of the Usedom and Wollin spits. The youngest sediments are lagoonary deposits, characterised by a most variable thickness: thin on the older terrace level, which occurs at higher altitude, while thick above the marine sands.
Acknowledgements The authors wish to express their gratitude to the team of Operational Oceanography from the Maritime ! Institute in Gdansk for accomplishing geophysical survey and coring works in 1998 and 1999. We also thank our students from the Institute of Marine Sciences, Faculty of Natural Sciences of the University of Szczecin. This in particular is directed to Krzysztof Gusar, Dariusz Sikorski, Dariusz Wereszka and Robert ! Wozinski for their help in coring works in the Great Lagoon and Dgabie Lake during winter 1995/96 and 96/ 97. The authors are grateful to Dr. Wolfram Lemke from the Baltic Sea Research Institute in Warnemunde . for critical reading of the manuscript. The study was financed by Committee on Scientific Research (KBN), Warsaw, Poland within the framework of project 6PO4E 00 216.
References . Bjorck, S., 1995. A review of the history of the Baltic Sea, 13.0–8.0 ka BP. Quaternary International 27 (1), 19–40. ! Borowka, R.K., 2001. Stratigraphical variability of the chemical composition of deposits infilling the Szczecin Lagoon and Da) bie Lake. In: Florek, W. (Ed.), Geology and Geomorphology of the Baltic Sea Southern Coast, Vol. 4. Pomorska Akademia Pedagogiczna, S"upsk, pp. 9–24 (in Polish). ! Borowka, R.K., Duda, T., 2002. A locality of the Holocene marine deposits in the Lower Odra Valley in Szczecin, in press.
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! ! Borowka, R.K., Gonera, P., Kostrzewski, A., Zwolinski, Z., 1982. Origin, age and palaeogeographical significance of cover sands in the Wolin end moraine area, North West Poland. Quaestiones Geographicae 8, 19–36. ! Borowka, R.K., Gonera, P., Kostrzewski, A., Nowaczyk, B., Zwoli! nski, Z., 1986. Stratigraphy of eolian deposits in Wolin Island and the surrounding area, North-West Poland. Boreas 15, 301–309. ! ! Borowka, R.K., Belczynska, A., Duda, T., Babski, K., 1999a. Postglacial development of the Wollin end moraine in the light of ! lithostratigraphic analysis of cover deposits. In: Borowka, R.K. (Ed.), Geology, Hydrogeology and Environmental Protection of the Western Pomerania coast. LXX Meeting of the Polish Geological Society. Szczecin, pp. 133–140 (in Polish). ! ! Borowka, R.K., Belczynska, A., Tomkowiak, J., 1999b. Morphological features and some chemical properties of fossil soils developed ! e)touj!scie and Grodno. within the aeolian sands in the vicinity of Swi ! In: Borowka, R.K., M"ynarczyk, Z., Wojciechowski, A. (Eds.), Evolution of the Southern Baltic Sea Coastal Geosystems. Bogucki ! Wydawnictwo Naukowe, Poznan-Szczecin, pp. 37–42 (in Polish). ! Borowka, R.K., Witkowski, A., Tomkowiak, J., Olas, M., Gusar, K., Liszkiewicz, E., 1999c. Stratigraphic and spatial lithological and geochemical variability of sediments in the Szczecin Haff and ! adjacent basins. In: Borowka, R.K., M"ynarczyk, Z., Wojciechowski, A. (Eds.), Evolution of the Southern Baltic Sea Coastal ! Geosystems. Bogucki Wydawnictwo Naukowe, Poznan-Szczecin, pp. 55–62 (in Polish). ! ! Borowka, R.K., Wawrzyniak-Wydrowska, B., Witkowski, A., Wozinski, R., 2000. Holocene molluscs assemblages in the deposits of the Gulf of Szczecin. Folia Malacologica 8 (4), 287. ! Borowka, R.K., Osadczuk, A., Duda, T., Pazdur, A., Pawlyta, J., 2001. Lithostratigraphy and chronostratigraphy of the infilling deposits the Szczecin Lagoon, Northwestern Poland. Seventh International Conference ‘‘Methods of Absolute Chronology’’ ! Poland. pp. 71–72 (Book of 23–26th April 2001, Ustron, Abstracts). . Bramer, H., 1978. Uber Transgressionsvorg.ange am Sudrand . des Kleinen Haffs. Wissenschaftliche Zeitschrift der E.-M. ArndtUniversit.at Greifswald, Jahrgang 27, Mathematisch-naturwissenschaftliche Reihe Heft 1/2, pp. 11–15. Broszinski, A., 2002. Die subfossile Diatomeenflora der westlichen Ostsee. Biostratigraphische Untersuchungen an sp.at- und postglazialen Sedimenten des Kattegats, der Kieler Bucht und der Pommerschen Bucht. Ph.D. Thesis, J.-W. Goethe University Frankfurt am Main. Duda, T., 1999. Phytogenic deposits in the Lower Odra Valley: their ! thickness, formation, lithology and age. In: Borowka, R.K., M"ynarczyk, Z., Wojciechowski, A. (Eds.), Evolution of the Southern Baltic Sea Coastal Geosystems. Bogucki Wydawnictwo ! Naukowe, Poznan-Szczecin, 73–78 (in Polish). Florek, W., 1996. Late Vistulian and Holocene development of the North Pomeranian river valleys and the influence of South Baltic neotectonics. Zeitschrift fur . Geomorphologie 102 (Suppl.- Bd.), 169–183. Karczewski, A., 1968. Inflow of the Odra lobe recession on origin and development of valley system of the My!sliborskie Lakeland and ! ! Nauk, Szczecin Lowland. Poznanskie Towarzystwo Przyjacio" Prace Komisji Geograficzno-Geologicznej 8, 1–106. Keilhack, K., 1912. Die Verlandung der Swinepforte. Jahrbuch . der Koniglich Preussischen Geologischen Landesanstalt 32 (2), 209–244. Kliewe, H., Janke, W., 1991. Holoz.aner Kustenausgleich . im sudlichen . Ostseegebiet bei besonderer Berucksichtigung . der Boddenausgleichskuste . Vorpommerns. Pettermans Geographische Mitteilungen 135 (1), 1–14. Kolp, O., 1990. The Ancylus Lake phase of the Post-Glacial evolution of the Baltic Sea. Quaestiones Geographiceae 13/14, 69–86.
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Kozarski, S., 1983. River channel adjustment to climatic change in west central Poland. In: Gregory, K.J. (Ed.), Background to Palaeohydrology: a perspective. Wiley, Chichester, pp. 355–374. Kozarski, S., Rotnicki, K., 1977. Valley floors and changes of river channel patterns in the North Polish Plain during the Late Wurm . and Holocene. Quaestiones Geographiceae 4, 51–93. Kramarska, R., 1999. Quaternary Geology of the south-western Baltic ! Sea. In: Borowka, R.K. (Ed.), Geology, Hydrogeology and Environmental Protection of the Western Pomerania coast. LXX Meeting of the Polish Geological Society. Szczecin, pp. 43–52 (in Polish). Lata"owa, M., 1999a. Hydrological changes caused by the climatic factor and human activity and their record in the Ko"czewo ! peatbog and lacustrine deposits of the Racze Lake. In: Borowka, R.K., M"ynarczyk, Z., Wojciechowski, A. (Eds.), Evolution of the Southern Baltic Sea Coastal Geosystems. Bogucki Wydawnictwo ! Naukowe. Poznan-Szczecin, pp. 99–103 (in Polish, English summary). Lata"owa, M., 1999b. Late Vistulian vegetation on Wolin Island ! (NW Poland)—the preliminary results. In: Borowka, R.K. (Ed.), Quaternary Studies in Poland, Special Issue, pp. 147–156. Leipe, T., Neumann, T., Emeis, K.-Ch., 1995. Schwermetallverteilung in holoz.anen Ostseesedimenten. Geowissenschaften 13 (12), 470–478. Leipe, T., Eidam, J., Lampe, R., Meyer, H., Neumann, T., Osadczuk, A., Janke, W., Puff, T., Blanz, T., Gingele, T.X., Dannenberger, D., Witt, G., 1998. Das Oderhaff. Beitr.age zur Rekonstruktion der holoz.anen geologischen Entwicklung und anthropogenen Beein-
. flussung des Oder-Astuars. Institut fur . Ostseeforschung Warnemunde. . Meereswissenschaftliche Berichte 28, 1–61. Mojski, J.E. (Ed.), 1995. Geological Atlas of the Southern Baltic. Polish Geological Institute. Sopot-Warszawa. Muller, . A., 2001. Late- and Postglacial Sea-Level Change and Paleoenvironments in the Oder Estuary, Southern Baltic Sea. Quaternary Research 55 (1), 86–96. Prusinkiewicz, Z., Nory!skiewicz, B., 1966. Problem of age of podsols ! on brown dunes of Bay Bars of River Swina in the light of a palynological analysis and dating by radiocargon 14C. Zeszyty Naukowe Uniwersytetu Miko"aja Kopernika w Toruniu Geografia 5, 75–88 (in Polish). . Rosa, B., 1963. Uber die morphologische Entwicklung der Kuste . Polen im Lichte der alten Strandformen. Studia Societatis Scientiarum Torunensis 5, 1–172. Rotnicki, K., 1983. Modelling past discharges of meandering rivers in the North Polish Plain in the Late Glacial and Holocene. In: Gregory, K.J. (Ed.), Background to Palaeohydrology: A Perspective. Wiley, Chichester, pp. 321–354. Rotnicki, K., 1991. Retrodiction of palaeodischarges of meandering and sinuous alluvial rivers and its Palaeohydroclimatic implications. In: Starkel, L., Gregory, K.J., Thornes, J.B. (Eds.), Temperate Palaeohydrology. Wiley, Chichester, pp. 431–471. Svensson, N.-O., 1991. Late Weichselian and Early Holocene shore displacement in the Central Baltic Sea. Quaternary International 9, 7–26. Wypych, K., 1980. Bottom deposits of the Szczecin Lagoon. In: Majewski, A. (Ed.), Szczecin Lagoon. Instytut Meteorologii i Gospodarki Wodnej. Wydawnictwa Komunikacji i Ła) czno!sci. Warszawa, pp. 54–72 (in Polish).
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Late Glacial and Holocene depositional history in the eastern part of the Szczecin Lagoon (Great Lagoon) basin—NW Poland ! Ryszard K. Borowka*, Andrzej Osadczuk, Andrzej Witkowski, Brygida Wawrzyniak-Wydrowska, Tomasz Duda University of Szczecin, Institute of Marine Sciences, Wa) ska 13, Pl–71-415 Szczecin, Poland Available online 2 July 2004
Abstract Analyses on 27 sediment cores taken from the bottom of the Szczecin Lagoon allowed environmental reconstruction of the postglacial main stages of basin development, based on detailed sedimentological, geochemical, diatomological and malacological studies of selected key cores. Studies revealed that during the Late Glacial and Holocene this area developed in several stages. In the Late Glacial the whole study area constituted a low alluvial plain. At the turn from Younger Dryas to Holocene the alluvial plain was cut through by the Odra river to a level of 10–11 m below sea level (b.s.l.). Along with the first phases of the Holocene marine transgression at the southern Baltic Sea’s coasts the accumulation of the limnic-swampy deposits began in this lower part of the Odra valley. At ca. 6–6.5 ka BP the transgression proceeded and Littorina Sea waters flooded the area. At that time the Szczecin Lagoon constituted a marine embayment in which series of sands, partly rich in malacofauna, was deposited. The development of the Swina barrier resulted in the isolation of the embayment from the direct inflow of Baltic Sea waters. r 2004 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Holocene development of the southern Baltic Sea shows distinct differences in comparison to the other areas of the Baltic, e.g. isostatic rebound processes at the Scandinavian coasts, which are absent in the Southern . Baltic Sea (e.g. Svensson, 1991; Bjorck, 1995). Thus the southern coasts of the Baltic Sea usually show sedimentary sequences reflecting marine transgressions. Beginning from the onset of the Boreal and Atlantic Chronozones the global eustatic sea level rise caused an inflow of marine waters. This resulted in changes of the water chemistry and in a general water level rise. The rising water level led to flooding of the former terrestrial (lacustrine) environment and the shores were translocated towards south, beginning from ca. 7000 BP. This sequence of events is typical for the Pomeranian Bay and the neighbouring basins of the lower Odra Valley and the Szczecin Lagoon (Kolp, 1990; Kliewe and Janke, 1991; Kramarska, 1999; Broszinski, 2002). The Szczecin Lagoon (Fig. 1) together with the Great Da) bie and Small Da) bie lakes to the south constitutes a *Corresponding author.
system of extensive sedimentary basins in which sedimentation of allochthonous and autochthonous deposits took place during most of the last millenia. These deposits result from terrigenous fluvially transported material and in situ produced biogenous and chemogenous sediments. The major source of the terrigenous material was the Odra River. However, during storm-induced high levels of the Pomeranian Bay also the Swina, Dziwna and Peene Rivers transported sediments into the lagoon. The Szczecin Lagoon area is primarily the valley floor of the deglacial Odra River valley which, due to the post-glacial transgression, was transformed into a lagoon when the spits along the present coast of the Baltic Sea were formed. Earlier geological studies of the Szczecin Lagoon ! (e.g. Wypych, 1980; Leipe et al., 1998; Borowka et al., 1999a–c; Muller, . 2001) were principally based on analyses of solitary and often randomly located cores. The present paper is based on the results of a comprehensive geophysical survey and coring programme of the Lagoon, aiming towards the recognition of depositional sequences with a time span from lateglacial to the present.
1040-6182/$ - see front matter r 2004 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2004.04.034
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Fig. 2. Enlarged section of the seismoacoustic profile from the NE part of the study area (for location see Fig. 1).
malacological and geochemical analyses and for 14C dating. Detailed information on 14C dating is provided in Table 1.
3. The study area
Fig. 1. Location of the study area. Indicated are positions of the seismoacoustic profiles and of vibrocores.
2. Materials and methods Seismoacoustic studies of the Szczecin Lagoon sediments were done by means of the sub-bottom profiler ‘‘Seabed Oretech 3010-S’’ at 5 kHz. Altogether 18 seismo-acoustic profiles, 200 km in total length, were accomplished (Figs. 1 and 2). Based on these results 27 sites were chosen for vibrocoring (Fig. 1). Vibrocores were taken by means of a VKG-3 device, 4 m long and 9.1 cm of internal diameter. The cores 3–4 m in length were cut into 1 m intervals and stored at a temperature of 3–4 C. In laboratory conditions all cores were documented photographically and their lithology was described macroscopically. Included in the description were the colour of the sediments, structural and textural features, organic matter content and the presence of fossil macro-remnants. Sub-sampling was done for granulometric, diatomological, palynological,
The investigations were carried out in the eastern part, deep-water part of the Szczecin Lagoon. The Szczecin Lagoon is a shallow water body with a surface area of 687 km2. The recent depth of the lagoon does not exceed 8.5 m and has an average of c. 3.5 m. Almost half (48.3%) of the lagoonary surface area has water depths ranging from 4 to 6 m. The northern boundary of the lagoon, which also separates it from the Baltic Sea, is the Swina Bar. This bar complex is made up of Holocene beach deposits and deltaic deposits of the Swina River. In the south the lagoon is gradually getting narrower (Fig. 1) forming the ! so-called Roztoka Odrzanska (Odra mouth) and finally the Lower Odra Valley.
4. Results The total number of studied cores amounted to 27. Results of these studies show that the sedimentary infill of the Szczecin Lagoon is composed of several depositional units, indicating different and changing depositional environment. The following depositional series can be distinguished in stratigraphic order: fluvial, limnic-swampy, marine and lagoonal. The best examples of this development and succession were observed in cores 35/99, 42/99 and 3/96 (Figs. 3 and 4).
Table 1 Radiocarbon dates of limnic and swampy deposits from the Szczecin Lagoon Sample name
(2/98/0.64–0.75) (2/98/0.75–0.81) (9/96/97–100) 26/56 26/65 26/69 28/57 28/59–60 29/26 29/31 35/41–42 35/46 35/50 36/23–24 36/27 36/48 37/53 39/61–62 42/30 42/36 42/41 42/44 42/45 43/10 43/12–13 43/32–33 47/34–35 47/48–49
Material
Radiocarbon age (14C yr B.P.)
Calibrated age (cal yr B.P.)
Calibrated age range (1 sigma) (cal yr B.P.)
Laboratory number
2/98 2/98 9/96 26/99 26/99 26/99 28/99 28/99 29/99 29/99 35/99 35/99 35/99 36/99 36/99 36/99 37/99 39/99 42/99 42/99 42/99 42/99 42/99 43/99 43/99 43/99 47/99 47/99
0.64–0.75 0.75–0.81 4.90–5.00 2.75–2.80 3.20–3.25 3.40–3.45 2.80–2.85 2.90–3.00 1.24–1.30 1.49–1.53 1.92–2.00 2.15–2.20 2.35–2.40 1.10–1.20 1.30–1.33 2.82–2.86 2.62–2.66 3.00–3.10 1.50–1.55 1.80–1.86 2.00–2.03 2.10–2.14 2.14–2.18 0.44–0.50 0.55–0.63 1.51–1.61 1.66–1.75 2.35–2.45
Peat Peat Organic mud Detritus gyttja Mudy gyttja Organic mud Organic mud Detritus gyttja Organic mud Detritus gyttja Peat Peat Detritus gyttja Peat Peat Organic mud Organic mud Detritus gyttja Detritus gyttja Detritus gyttja Peat Peat Muddy peat Detritus gyttja Detritus gyttja Organic mud Detritus gyttja Muddy gyttja
74107140 74307140 72507130 72407120 79907110 73307110 62307250 77007120 7050790 69307110 7030790 88107110 104207180 78707100 71707110 111007130 119007230 81807160 6850790 7230780 100307120 114507140 101807380 70807100 77707130 99807110 7260775 78507100
82357125 82657115 80607130 80607120 88507160 80907120 71257325 84907120 78557105 77607100 78507100 98757275 123007350 86657125 79507110 130557145 138757425 91007350 7685775 8100790 115007250 133507200 119507700 78807100 85557165 114357205 80707100 87257275
8360–8110 8380–8150 8190–7930 8180–7940 9010–8690 8210–7970 7450–6800 8610–8370 7960–7750 7860–7660 7950–7750 10150– 9600 12650–11950 8790–8540 8060–7840 13200–12910 14300–13450 9450–8750 7760–7610 8190–8010 11750–11250 13550–13150 12650–11250 7980–7780 8720–8390 11640–11230 8170–7970 9000–8450
Gd-15096 Gd-12233 Gd-15092 Gd-15103 Gd-12236 Gd-15101 Gd-16027 Gd-12234 Gd-12241 Gd-12244 Gd-12223 Gd-12224 Gd-15090 Gd-12242 Gd-12243 Gd-12245 Gd-15086 Gd-15078 Gd-12217 Gd-12222 Gd-12214 Gd-12220 Gd-16029 Gd-12235 Gd-15104 Gd-12238 Gd-12225 Gd-12226
6.64–6.75 6.75–6.81 9.90–10.00 8.70–8.75 9.15–9.20 9.35–9.40 8.52–8.57 8.62–8.72 6.96–7.02 7.21–7.25 6.93–7.01 7.16–7.21 7.36–7.41 7.05–7.15 7.25–7.28 8.77–8.81 8.48–8.52 8.77–8.87 7.55–7.60 7.85–7.91 8.05–8.08 8.15–8.19 8.19–8.23 5.76–5.82 5.87–5.95 6.83–6.93 7.58–7.67 8.27–8.37
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Fig. 3. Core logs 35/99 and 42/99: 1—shell layer; 2—algal gyttja with shells; 3—algal gyttja; 4—mineral gyttja; 5—silty gyttja; 6—sandy gyttja; 7— detritus gyttja; 8—peat; 9—sand; 10—silty sand.
Fig. 4. Core log 3/96 including results of the malacological analysis (for explanations see Fig. 3).
4.1. Late Glacial sediments 4.1.1. Unit A: fluvial environment The basement of the organic deposits filling in the Szczecin Lagoon basin is of fluvial origin. The coring equipment could not penetrate the full thickness of this unit but a data from older studies (e.g. Wypych, 1980) indicate that it reaches at least the depth of 24 m b.s.l. The total thickness could thus reach 14–16 m, possibly
with the lower part representing fluvio-glacial deposits. The maximum penetration at coring these sediments was 3.5 m. The sediments are well- to very well-sorted fine to medium sands, in the lower parts sometimes showing cross bedding. In the uppermost part the sand is more fine-grained and the beds show a horizontal lamination. Within some of the cores (e.g. 35/99; 42/99 and 47/ 99), the upper part of the unit A sands display numerous
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root horizons and charcoal intraclasts. In core (36/99, 37/99 and 42/99) the unit A fluvial sand is interbedded with a thin subunit (10–20 cm) of organic-rich, clayeymuddy sediments, interpreted to be of limnic-swampy origin. 14C dates (core 42/99) of this layer indicate that it . age. is of Allerod The morphology of the upper surface of the unit A fluvial sediments is variable (Fig. 5A). Its deepest position was recorded in the middle part of the Great Lagoon, where it appears at 10–11 m b.s.l. and in the north-western part, close to the border of the Little Lagoon. In the eastern and western part of the coast of the Great Lagoon the level of the upper boundary rises to 6–7 m b.s.l. The analysis of the seismo-acoustic data thus reveals the presence of an elongated basin with a relative height of up to c. 3 m within the upper surface of the fluvial sediments, which is interpreted as an erosional channel related to the lateral migration of the Pre-Odra River.
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4.2. Holocene sediments 4.2.1. Unit B: Limnic-swampy environment Organic deposits, though absent in some cores, drape the unit A top surface with a sediment thickness between 10 cm and 2 m. Spatial analysis of its thickness distribution shows that these sediments primarily fill the elongated depressions within the upper boundary of the fluvial unit A sediments, particularly in the middle part of the Great Lagoon (Fig. 5B). They are interpreted as ancient river beds of the Pre-Odra River. The unit B sediments are predominantly composed of gyttja, sometimes underlain or overlain by well decomposed peat with wood pieces. The organic matter content of the gyttja range from 20% to 80%, sometimes with a distinct portion of CaCO3. The gyttja display a freshwater molluscan fauna with the dominant taxa being characteristic for small lacustrine basins, e.g. Bithynia tentaculata, Valvata piscinalis, Lymnaea sp. and
Fig. 5. Szczecin Lagoon. Morphology of the top surface of the fluvial (Unit A), swampy-limnic (Unit B), brackish-marine (Unit C) and lagoonary series (Unit D).
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Pisidium sp. Some fish remnants were also sporadically identified within the sediment. An abundant diatom flora was recorded (Fig. 6) predominated by the epiphytic taxa representing genera of Cocconeis, Epithemia, Fragilaria and Gomphonema. Their predomination indicate the existence of shallow freshwater bodies with abundant macro-plants. Furthermore, the dominance of benthic taxa also indicates a shallow water environment with good light conditions. The frequency (up to 6%) of brackish-water taxa increases in the upper part of the unit, e.g. represented by Fragilaria schulzii, Navicula salinarum and Melosira nummuloides. In more shallow parts of the basin, unit B is represented only by peat layers up to ca. 2 m in thickness. Peat type varies, with carex and carex-reedy peat in the basal part of the profile and woody and woody-carex peat in the upper part. Based on sediment types and macro-fossil content, unit B is interpreted as deposited in a varying swampy to limnic environment. The deposition started as indicated from 14C dating (Fig. 3), in the beginning of the Holocene and lasted up ! to ca. 6200 BP (Borowka et al., 2001). Existing data indicate their formation to start after a distinct environmental shift from fluvial conditions to shallow limnic conditions. Only one core (43/99, Fig. 1) records fluvial sediment interbedding (c. 1 m sands) within the unit B limnic sediments. 4.2.2. Unit C: marine environment The base of the unit C sediments is marked by a distinct erosional discontinuity and occurs in almost all cores taken from the deeper part of the Great Lagoon. They are characterised by increased contents of the coarse fraction and relatively good sorting. The thickness of the marine unit C sediment is variable, but it rarely exceeds 1.5 m. Its greatest thickness was recorded in the northern part of the Great Lagoon (Fig. 5C); in core 31/99 (Fig. 1) it attained ca. 3 m without being
totally penetrated. The marine sediments were not encountered in the lateral, more highly siltated parts of the basin. With respect to lithology, the marine sediments are composed of sands and muddy sands. They differ from the unit A fluvial sands with respect to organic matter, Na, K, Ca, Mg and Fe contents (Fig. 6), finer grain-size and less sorting. The results from the malacological analyses are highly indicative of a marine environment. The mollusc fauna of the sandy sediments is dominated by Cardium glaucum, forming well developed populations with shell sizes larger than in the recent Pomeranian Bay. Accompanying species are Hydrobia ventrosa, H. ulvae and H. balthica (Fig. 4). A characteristic feature of the marine sediments is either a complete absence or a scarce, usually highly fragmented diatom flora (Fig. 7). This might be related to a high sedimentation rate of the sandy sediments and to current and wave action. The taxa found, e.g. Catenula adhaerens and Fragilaria fasciculata, are typical for a shallow marine littoral environment. 4.2.3. Unit D: Lagoon environment This unit differs from the marine unit with respect to a higher content of organic matter, to their chemical composition and the species composition of the molluscan fauna. Locally, its thickness exceeds 2 m in the middle part of the Great Lagoon, but it is very thin in the western part (Fig. 5D). The subfossil malacofauna of the lagoonary sediments is predominantly composed of freshwater taxa including Bithynia tentaculata, Valvata piscinalis, Viviparus sp., Pisidium sp., Unio sp., and Dreissena polymorpha. Presently, the latter species forms extensive colonies on some localities of the Great Lagoon bottom (e.g. core 35/99). The diatom flora (Fig. 7) is dominated by freshwater planktonic taxa such as Aulacoseira
Fig. 6. Distribution of diatom ecological groups in core No. 3/96. Solid area stands for %, dotted area stands for %.
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Fig. 7. Distribution of geochemical elements in core No. 3/96.
granulata, A. islandica and A. muzzanensis. Within the youngest deposits halophilous and brackish-water taxa appear, attaining percentages up to 9.6% and represented by taxa such as Diploneis smithii, Fragilaria sopotensis and Opephora olsenii. The lagoonary sediments show a distinct bipartition with respect to geochemical analyses (Fig. 6). Its upper part, up to half a meter in thickness, is characterised by markedly increased concentrations of heavy metals (Zn, Pb, Cu) and Mn. In comparison to the lower part there is a nine-fold increase in Zn and an almost five-fold increase in Cu content.
5. Discussion The analysis of the stratigraphic and spatial variability of the Great Lagoon sedimentary infill enables the distinction of several development stages of this depositional basin. Their chronostratigraphic position is based on numerous radiocarbon dates of the limnic-swampy ! deposits (Borowka et al., 2001). A summary of the paleogeographic development is presented in Fig. 8. . stage is represented by fluvial The Pre-Allerod sediments (unit A). Their origin is related to the development of the lower Odra River valley from the end of the last glaciation to the distinct climate . Within this period, a amelioration during the Allerod. series of six terrace levels in the area of the recent Odra River valley and the Szczecin Lagoon was formed. The highest is raised up to ca. 20 m a.s.l., whereas the lowest one is found at ca. 10 m b.s.l. (Karczewski, 1968; Duda, 1999). The age of particular terraces is not known yet. However, within terrace sediments at an altitude of ca.
8 m b.s.l., there are thin intercalations of limnic-swampy . deposits, dated to the Allerod. It is hard to reconstruct the late glacial bottom of the Odra River Valley and the course of the river bed itself. Paleohydrological studies (Kozarski and Rotnicki, 1977; Kozarski, 1983; Rotnicki, 1983, 1991) carried out for the Odra River catchment area and some of its tributaries revealed, that during the Late Glacial, large meandering river beds were formed within the bottom of some of the contemporary valleys. Compared to the Early and Middle Holocene they discharged several times higher magnitudes of water (Rotnicki, 1983, 1991). However, it cannot be excluded that the lower Odra was a braided river, similar to other rivers draining the northern slope of Pomerania (Florek, 1996). This problem thus remains an open question. . stage, it is known that some parts of From the Allerod the Szczecin Lagoon were covered by shallow water bodies and peat bogs (Fig. 8), giving rise to sediments interfingering with the predominantly fluvial A sediments. Simultaneously the surroundings of the Szczecin Lagoon were characterised by rather dense pine-birch forests and by patches of heliophilous plants. This is demonstrated not only from the results of paleobotanical analyses (Lata"owa, 1989, Lata"owa, 1999a, b), but also indicated by well developed and widely distributed ! covers of fossil soils on Wolin Island (Borowka et al., 1982, 1986, 1999a, b). The Younger Dryas stage is marked by a distinct acceleration of fluvial processes, and particularly by an aggradation of sandy deposits at earlier developed . limnic-swampy deposits in the Great covers of Allerod . Lagoon area. In cores 36/99 and 37/99, post-Allerod fluvial deposits attain a thickness exceeding 1.5 m. This
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Fig. 8. Szczecin Lagoon. Litho- and chronostratigraphy of the infilling deposits.
was probably related to the deterioration of climatic conditions during Younger Dryas, causing the disappearance of forest stands and acceleration of soil erosion processes (Lata"owa, 1999a, b). There are also indications on the intensification of periglacial and aeolian ! processes (Borowka et al., 1999a, b). During that period there was probably a distinct increase in the sediment load, transported by the Odra River. The Early and Middle Holocene stages encompass the time span from the beginning of the Preboreal to the second half of the Atlantic period (B10,250–6200 yr BP) at which the Great Lagoon area was predominantly filled in with limnic-swampy deposits (unit B; Fig. 8). It is highly likely that during that time the Pre-Odra was an anastomosing river, flowing through swamps and reedy areas, similar to the recent lower Odra.
The Late-Atlantic stage is marked by a phase of marine transgression (the Littorina transgression) in the Great Lagoon area. Almost the whole lagoon constituted an open marine bay at that time. It extended southward into the lower Odra River valley up to a place where the recent town Szczecin is located (Bor! owka and Duda, 2002) and penetrated the mouths of smaller rivers discharging into the Szczecin Lagoon, e.g. Uecker valley in the area of Ueckermunde . (Bramer, 1978). The water salinity was higher than in the present Pomeranian Bay, as indicated by preserved shells of Cardium glaucum, reaching sizes characteristic for water ! salinity values higher than 6–7% (Borowka et al., 2000). The duration and rate of this transgression in the area of the Pomeranian Bay and Szczecin Lagoon is not satisfactorily known. It cannot be excluded that it was a disastrous event, as suggested by Rosa (1963). It is possible that, during extremely strong storms, a sand bar between the Odra Bank and eastward up to the region of Ko"obrzeg was disrupted and destroyed (Mojski, 1995; Kramarska, 1999). Radiocarbon dates of limnic-swampy deposits from the Pomeranian Bay (Kramarska, 1999) and from the Szczecin Lagoon ! (Wypych, 1980; Borowka et al., 2001), showing very similar ages of these deposits in both areas, indicate a relatively fast rate of this transgression. The Late-Holocene stage, encompassing the Subboreal and the Subatlantic periods began with the isolation of the Szczecin Lagoon from direct marine influences. Intensified abrasion processes at morainic cliffs of Usedom and Wolin Islands resulted in a rapid growth of bars and the development of a sand barrier in the area of the recent Swina Gate (Keilhack, 1912; Prusinkiewicz and Norys!kiewicz, 1966). Simultaneously the formation of the Swina back-delta started behind the barrier. Its submerged part, the so-called Wyskok Krzecki, extends far into the lagoon’s interior and is dipping with a relatively steep slope to the depth of 6.5–7 m. The isolation of the Great Lagoon from direct marine influences resulted in a change of the depositional processes. This is indicated in the deposited sediments (unit D, Fig. 8) by an increase of the organic matter content and the replacement of marine molluscs by ! freshwater taxa (Borowka et al., 2000). Lagoonary sediments deposited during that time also contain a ! predominantly freshwater diatom flora (Borowka et al., 1999). The accumulation of this type of deposits has continued in the Great Lagoon until present times. The sedimentation rate is highly variable and ranges from ca. 20 cm per century in the Little Lagoon (Leipe et al., 1995) to ca. 50 cm in the eastern part of the Great ! Lagoon (Borowka, 2001). No deposition or very low deposition rates are characteristic for regions neighbouring the western coast of the Great lagoon, which are located on a distinct plateau on top of the fluvial unit A sediments.
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6. Concluding remarks The present geological studies have shown that the sediments filling the deep-water part of the Szczecin Lagoon are deposited on two morphological levels. . lacusOlder sediments, which encompass (e.g.) Allerod trine series are located on the higher level, c. 7–8 m b.s.l. Limnic and swampy sediments were initially deposited only at the lower terrace within the period of 10 to 6–6.5 ka BP. Later on, at ca. 8 ka BP, the swampy sedimentation inundated also the higher level. Beginning from this time the whole Great Lagoon constituted a vast and flat swampy plain, cut by the anostomosing Odra River. At c. 6–6.6 ka BP the whole low lying area was affected by a marine transgression and for a short period an open marine embayment existed, characterised by salinity values higher than the recent Pomeranian Bay. However, this embayment was rapidly isolated from the direct inflows of the Baltic Sea due to build-up of the Usedom and Wollin spits. The youngest sediments are lagoonary deposits, characterised by a most variable thickness: thin on the older terrace level, which occurs at higher altitude, while thick above the marine sands.
Acknowledgements The authors wish to express their gratitude to the team of Operational Oceanography from the Maritime ! Institute in Gdansk for accomplishing geophysical survey and coring works in 1998 and 1999. We also thank our students from the Institute of Marine Sciences, Faculty of Natural Sciences of the University of Szczecin. This in particular is directed to Krzysztof Gusar, Dariusz Sikorski, Dariusz Wereszka and Robert ! Wozinski for their help in coring works in the Great Lagoon and Dgabie Lake during winter 1995/96 and 96/ 97. The authors are grateful to Dr. Wolfram Lemke from the Baltic Sea Research Institute in Warnemunde . for critical reading of the manuscript. The study was financed by Committee on Scientific Research (KBN), Warsaw, Poland within the framework of project 6PO4E 00 216.
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